I sometimes shake when I'm not doing anything.
We all have different abilities and we all begin to lose some of those as we age. I guess I can tell you all kinds of games to play saying "this is the way to tell", but really, if you are questioning it, maybe you are.
Also. . . important. . . are you using your back ( I mean. . really using it ) ? I used to think I was and I felt the urge to shake on full draw. Then I actually saw the light ( and learned how to use my back and there is really very little pressure on my arms now.
I myself, don't normally shake at full draw.
ChuckC